1. Field of the Invention
The present invention relates to an LED drive circuit and an LED illumination component using the same.
2. Description of the Prior Art
An LED (light-emitting diode) is characterized by its low current consumption, long life, and so on, and its range of applications has been expanding not only to display devices but also to illumination apparatuses and the like. An LED illumination apparatus often uses a plurality of LEDs in order to attain desired illuminance (see, for example, JP-A-2004-327152, JP-A-2006-319172, and JP-A-2008-104273).
A general-use illumination apparatus often uses a commercial alternating current power source, and considering, for example, a case where an LED illumination component is used in place of a general-use illumination component such as an incandescent lamp, it is desirable that, similarly to a general-use illumination component, an LED illumination component also be configured to use a commercial alternating current power source.
Furthermore, in seeking to perform light control of an incandescent lamp, a phase-control light controller (referred to generally as an incandescent light controller) is used in which a switching element (generally, a thyristor element or a triac element) is switched on at a certain phase angle of an alternating current power source voltage and that thus allows light control through control of power supply to the incandescent lamp to be performed easily with a simple operation of a volume element (see, for example, JP-A-2005-26142). It is known, however, that in performing light control of a low-wattage incandescent lamp by use of a phase-control light controller, connecting the incandescent lamp to the light controller leads to the occurrence of flickering or blinking, so that the light control cannot be performed properly.
It is desirable that in seeking to perform light control of an LED illumination component that uses an alternating current power source, the LED illumination component be connectable as it is to an existing phase-control light controller for an incandescent lamp. By changing only an illumination component to an LED illumination component while using existing light control equipment therewith, compared with a case of using an incandescent lamp, power consumption can be reduced considerably. Furthermore, this can also secure compatibility without requiring the light control equipment to be changed to a type used exclusively for an LED illumination component and thus reduces equipment cost.
Now, FIG. 16 shows a conventional example of an LED illumination system capable of performing light control of an LED illumination component that uses an alternating current power source. An LED illumination system shown in FIG. 16 includes a phase-control light controller 200, an LED drive circuit 300 having a diode bridge DB1 and a current limitation portion IL, and an LED array 400 formed by connecting LEDs in series. The phase-control light controller 200 is connected in series between a commercial power source 100 that is an alternating current power source and the current limitation portion IL. In the phase-control light controller 200, a resistance value of a variable resistor Rvar is made to vary, and a triac Tri is thus switched on at a power source phase angle depending on the resistance value. Typically, the variable resistor Rvar is built in the form of a rotary knob or a slider and so configured that changing an angle of rotation of the knob or the position of the slider allows light control of the illumination component. Moreover, in the phase-control light controller 200, a capacitor Ca and an inductor L constitute a noise suppression circuit that reduces noise fed back into an alternating current power source line from the phase-control light controller 200. FIG. 17 shows output waveforms of the light controller and those of the diode bridge DB1, which correspond to phase angles of 0°, 45°, 90°, and 135° of the phase-control light controller 200, respectively. As the phase angle increases, an average value of a voltage of the output waveform of the diode bridge decreases. In a case where the LED illumination component is connected to the phase-control light controller 200, as the phase angle of the light controller increases, resulting brightness decreases.
When the phase angle of the phase-control light controller 200 is increased to decrease resulting brightness of the LEDs, if an output voltage of the diode bridge DB1 becomes smaller than a forward voltage (VF) obtained when the LED array 400 starts to glow, the LED array 400 no longer glows, and there occurs an abrupt decrease in current flowing through the light controller. Due to this abrupt decrease, the current flowing through the light controller falls below a level of an on-state holding current of the triac Tri in the light controller, so that the triac is switched off to halt the output of the light controller and thus to bring about an unstable state, which results in the occurrence of brightness flickering of the LED array 400. Furthermore, when the triac Tri is switched from an off-state to an on-state through phase control of the output of the light controller, the LEDs are switched from an off-state to an on-state, so that there occurs an abrupt variation in impedance of the LEDs. This might cause ringing to occur at an edge of an output voltage of the light controller, where the output voltage varies abruptly, so that the triac Tri is brought to an unstable state to be switched off, which results in the occurrence of brightness flickering. For the above-described reason, in an LED illumination system adapted to the use of a phase-control light controller, when LEDs are not glowing, a current drawing circuit that forcibly passes a holding current is used to prevent a triac from being switched off. In this case, however, a drawn current is all converted to heat, which leads to a deterioration in efficiency of the LED illumination system and also requires heat radiation measures to be taken.
In a case of using a conventional incandescent lamp load, since a filament of tungsten or the like constitutes the load, even if a triac of a phase-control light controller is switched from an off-state to an on-state, there hardly occurs a variation in impedance, and thus a low impedance state is maintained. Thus, there occurs no abrupt variation in current flowing through the phase-control light controller, so that a stable light control operation can be performed as long as an alternating current power source has a voltage value of around 0 V or higher.
Furthermore, in a case of the conventional example shown in FIG. 16, when an output voltage of the diode bridge DB1 is lower than a forward voltage (VF) obtained when the LED array 400 starts to glow, the LEDs are switched off, and assuming that the alternating current power source is at a frequency of 60 Hz, since full-wave rectification is performed by the diode bridge, the LEDs are switched on/off repeatedly at a frequency of 120 Hz that is double the alternating current power source frequency. This switching on/off of the LEDs causes flickering and might disadvantageously make it more likely that such flickering is perceived by a user when the user quickly moves his/her line of sight in an attempt to follow a quick move in a sporting event or the like. In a case of using an incandescent lamp, due to a filament that has a response speed on the order of 0.1 seconds and thus does not respond to an on/off operation at 120 Hz, flickering as described above does not occur to a noticeable degree. On the other hand, in a case of using an LED, since its response speed is a million or more times higher than that of a filament used in an incandescent lamp, flickering tends to occur to a noticeable degree.
Moreover, FIG. 18 shows a relationship (light control curve) between a phase angle θ of the phase-control light controller and illumination brightness in each of a case of the conventional LED illumination system shown in FIG. 16 and a case of an incandescent lamp illumination system. In the conventional LED illumination system, there occurs no variation in brightness at the phase angle θ=0° to 45°, while at θ=45° or larger, a light amount varies linearly, and at θ=130°, the LED illumination system is turned off. The incandescent lamp exhibits a curve characterized in that a light amount decreases mildly starting at θ=0°, which at θ=50° to 100°, varies along with the light control curve of the conventional LED illumination system and at θ=120° to 150°, decreases mildly. Brightness is perceived logarithmically by human eyes, and thus a characteristic that a light amount decreases mildly with respect to the phase angle θ is the key to fine control of a light amount at low illuminance. The conventional LED illumination system has been disadvantageous in that since it dims abruptly at around θ=130°, a light amount at a phase angle of around 120° to 150° cannot be controlled finely compared with a case of an incandescent lamp.